Cooking appliances such as hotplates and electric ranges typically have one or more surface unit heating elements for heating foods and liquids contained in utensils which are placed on the surface units. Frequently, such appliances are used to heat a liquid to its boiling point and possibly to maintain the liquid at that temperature for some period of time. Examples include boiling water for preparing tea, instant coffee and other hot beverages, as well as a step in food preparation such as boiling potatoes or cooking spaghetti or other pasta dishes. Typically, when faced with such a task, the user in an effort to bring the liquid to a boil as rapidly as possible will select the maximum power level for the heating element. This power setting selection generally accomplishes the intended result of bringing the liquid to boiling quickly. However, for most typical liquid loads, the energy delivered to the load, when the surface unit is operating at its maximum power setting, greatly exceeds that needed to maintain the liquid at the boiling temperature. This excess energy is absorbed as latent heat of vaporization, causing the liquid to boil violently with a high evaporation rate. In addition to being wasteful from an energy efficiency standpoint, continued operation of the surface unit at maximum power tends to boil the liquid away quickly, with the result that, if left unattended, the liquid may boil away completely, resulting in improperly cooked food and possibly damage to the utensil.
It is thus desirable to provide a power control arrangement which automatically adjusts the level of power applied to a heating unit downwardly once the liquid being heated reaches its boiling point to a power level sufficient to maintain the liquid at that temperature, but at a relatively minimal evaporation rate.
Additionally, when preparing foods which call for boiling the food for a particular period of time, typically the user must watch for the liquid as it is initially heated to visually detect when the boiling point has been reached to start the time period.
It is thus desirable to provide an arrangement for detecting when the liquid has begun to boil to provide a signal to the user that the boiling point has been reached. Such information could also be used to automatically begin the timing period for foods to be boiled for a predetermined time period.
It is therefore an object of the present invention to provide a method and apparatus applicable to a cooking appliance for controlling the power output of the appliance heater as a function of the weight of the food load being heated.
It is another object of this invention to provide a method and apparatus applicable to a cooking appliance which provides for repetitive measurement of the weight of the contents of a utensil being heated and controls the output power of the heater in accordance with the relationship of the rate of change of the measured weight to a reference rate.
It is another object of the present invention to provide a method and apparatus applicable to cooking appliances which detects that the boiling point of the liquid being heated has been reached and automatically reduces the power level of the heat source of the appliance when used to boil a liquid when the boiling point for the liquid has been reached, to the lowest power level which will sustain a desired evaporation rate.
It is another object of this invention to provide a method and apparatus applicable to a cooking appliance which automatically limits the evaporation rate of liquids boiled by the appliance to a desired rate by reducing the output power of the appliance heater when the rate of change of the measured weight signifies a rate of decrease or evaporation rate greater than the desired reference rate and which signals the detection of the first occurrence of such a decrease signifying that boiling of the liquid has begun.